1. Field of the Invention
The invention relates to a process for treating industrial wastewaters, in particular wastewaters from the metal-processing industry, preferably suitable for treating the wastewater in such a manner that it can be recycled back to the production.
2. Description of the Related Art
In the metal processing industry, frequently large amounts of wastewaters are produced, for example from the cutting or polishing of workpieces, in the form of cutting fluids or as cooling water or flushing water. Customarily, this wastewater is released after it is passed through an appropriate wastewater treatment plant (WTP). Multiple use or reuse of the wastewater in production is desirable for reasons of costs.
The abovementioned wastewaters are essentially polluted with the following pollutants (the basis used here is the German Wastewater Framework Administrative Provisions, Annex 40):
[t1]
These pollutants are currently dealt with using appropriate chemical or physical processes, as shown in the abovementioned table, and removed. It is necessary to comply here with the legally established concentrations. In particular, flocculation and/or precipitation of the pollutants plays a central role in the treatment process, since large amounts of water can only be treated using these processes. However, as a byproduct of the precipitation process, the water becomes salted with calcium compounds, sodium compounds, chloride compounds and sulfate compounds.
DE 37 09 174 discloses a process which describes cleaning up organically polluted wastewater. In the process the organic pollutants are microbiologically converted under superatmospheric pressure and the resultant biological sludge is separated from the wastewater by membrane filtration or ultrafiltration. However, this process has the disadvantage that poorly biodegradable organic pollutants or inorganic pollutants which are of a particle size below the membrane separation limit are not converted or are not removed, but remain in the wastewater.
EP 0 503 115, building on DE 37 09 174, describes a process in which not only the organic biodegradable pollutants, but also the poorly biodegradable or non-biodegradable organic pollutants and inorganic pollutants are removed from the wastewater. In this process the poorly biodegradable or non-biodegradable organic matter is physically and/or chemically treated so that it can be fed to a biological treatment. In addition, in particular the pollutants are concentrated and thus passed repeatedly through the biological purification. This concentration is carried out by membrane filtration, preferably nanofiltration, with nanofiltration having the advantage that only low-molecular-weight dissolved consistuents, especially salts, remain in the wastewater thus treated. However, this process also has the disadvantage that the treated wastewater is not further used, but released into the sewage system.
DE 38 15 271 discloses a process in which, before or after the biological purification stage, at least one of the following processes is carried out: adsorption, membrane filtration and/or oxidation. However, this process also has the disadvantage that the treated wastewater is not further used.
A process for repeated utilization of wastewaters is finally disclosed by DE. In this case, additional quality criteria for reuse of the wastewater in production are specified:
[t2]
After passing through a suitable WTP, the wastewater is first passed through a sand filter. The wastewater then passes through a fixed-bed reactor for COD breakdown. The Ca ions are then exchanged for Na ions in two water-softening columns operating alternately. The final nanofiltration then serves to set all quality parameters of the wastewater which is then thus purified. It must be ajudged here a disadvantage that in this process a relatively large amount of wastewater is produced and an additional wastewater stream is produced by the water-softening system.
Against this background, it is an object of the present invention to develop a process which continuously treats industrial wastewaters in such a manner that they are again available for production and the disadvantages of the above described processes are avoided.
This object is achieved by a process for treating industrial wastewaters by the method of the present invention. According to the present invention, the wastewater, preferably wastewater from the metal processing- industry, is first pretreated by a flocculation or precipitation process for removing the heavy metals from the wastewater. The flocculation aids used are customarily metal salts or combination products. The flocculation aids are added in a neutral to acid environment. The mixture is then neutralized using an aqueous alkali solution, preferably sodium hydroxide solution, milk of lime or the like being used. The resultant flocks are removed using a suitable separator, for example a tilted-plate clarifier. Depending on the type of the metal-processing operation, a subsequent oil separator or coalescence separator may be required, which then removes the free oil from the wastewater stream. Via a suitable filter system, for example a sand filter, to remove residual flocks and/or suspended matter, the wastewater then passes to a biological purification stage. This biological purification stage is generally constructed as trickling filter or fixed-bed reactor. The biological purification stage serves for decreasing the chemical oxygen demand (COD). Preferably, the biological purification stage is supported by injecting oxygen. Downstream of the biological purification stage, a fine filter can be connected which protects the nanofiltration membrane from carbon abrasion from the biological purification stage. To stabilize the temperature, a heat exchanger can be integrated which increases or decreases the temperature as required. Finally, the wastewater is fed into a single-stage or multistage nanofiltration system. Filter system, biological purification stage and nanofiltration system are backwashed as required in a known manner.